This invention relates generally to fire-fighting equipment, and more particularly to a system for accurately controlling the introduction of a liquid chemical foamant concentrate into a water stream.
Foamant delivery systems often employ fixed displacement pumps to deliver a measured quantity of concentrated liquid foaming agent, i.e., a foamant, to one or more discharge streams driven by a water pump associated with a firefighting vehicle or other apparatus. The foamant and water mix, preferably in accurate proportions, and produce a foam discharge that is directed onto a fire or onto combustible fuel.
In truck-mounted systems, pumps are usually driven by the truck motor, via a power take off arrangement. Mechanical pumps (e.g., a gear coupled to the truck motor), hydraulic motor power take-offs, and electrical pumps are all possible and a variety are conventionally known.
In a pump and foamant insertion arrangement, it is necessary to set and/or control and balance the flow conditions of the water and foamant in order to achieve a predetermined proportion of foamant volume to water volume. The pressure/flow conditions are advantageously monitored and controlled so as to maintain the balance and correct proportion, either by automatic control or manual adjustment. If the wrong proportion of foamant is used, the resulting foam may be less effective than it might be at suppressing a fire, and use of too much foamant can be a waste of money and/or can exhaust a limited supply of foamant before a fire has been extinguished.
For example, foamant concentrate supply conduits may lead from a foamant pump into a manifold for mixing. A foamant conduit may open into a water flow at a venturi. These and other devices can be used to discharge a concentrated foamant stream into a water conduit in series with a water pump. To maintain the proportion of foamant to water needed, the discharge flow rate of the foamant should be maintained at a percentage selected by an operator. One can assume that operation of the water pump produces a given water flow rate and then inject foamant at a rate that is proportional to operation of the pump. Proportioning valves controlled by the water flow rate (e.g., the rate of operation of the water pump) can be placed between the foamant concentrate supply conduits and the water discharge outlets. These valves operate either to selectively isolate the discharge outlets from the concentrate pump (i.e., in the closing direction) or to inject or increase the proportionate amount of foamant concentrate being fed into the water (in the opening direction).
An example of such a system in the prior art is disclosed in U.S. Pat. No. 5,232,052- Arvidson et. al. The Arvidson patent discloses a fire-fighting system in which a liquid chemical foamant is introduced into a pumped water stream. The foamant is added in varying amounts in an effort to maintain a predetermined percentage mixture of foamant concentrate volume to water volume, and to keep the percentage or proportion independent of variations in the water flow. Arvidson""s apparatus includes a motor-driven pump controlled over first and second speed ranges by pulse width modulation of the drive signal applied to the motor. An intermittent motion is produced, by burst modulating the drive signal, when the pulse width modulation is operating at a minimum duty cycle. Using this technique, the range of available motor speeds is extended beyond that which can be achieved using pulse width modulation of the motor drive signal alone.
Prior apparatus and methods for controlling flow when introducing liquid concentrate foaming agent into pumped water have often proved inefficient and wasteful as a means for dispensing the relatively expensive concentrate. Sensing volume flow rates and controlling an injection flow rate based on sensed flow is not a trivial problem. Pump operation, pressures, and flow rates can be interactive in that operational variations in certain parameters of operation can affect other parameters, sometimes affecting the particular parameter that might be used to infer a flow rate or to control the flow rate of foamant and/or water. As an example, a conventional system may provide a separate flow meter for determining the flow rate of foamant, or may assume a particular flow rate from the measured speed or from a driving signal applied to a positive displacement piston pump. A separate flow meter may add significantly to the expense of such as system, especially if multiple discharges and hence multiple flow meters and multiple control loops are needed. Moreover, the prediction of foam flow based only on pump speed is not accurate over a broad range of flows and pressures. Thus the added expense of the sensing and control loops is not the end of cost concerns. The relative inaccuracy and lack of precision involved are such that such systems also waste a certain amount of foaming agent.
The present invention addresses and improves upon a number of aspects of the prior art by providing a foam injection system applicable to a truck or other installation having many outlets, perhaps many diameters and a wide range of potentially required water and/or foam flow rates. In its broadest aspects, a liquid injection arrangement is provided that inserts a foaming agent into water for a fire fighting application, or a similar use requiring that metered quantities of a liquid chemical foamant or other agent be injected into a conduit assembly conveying a water stream. A desired concentration of foamant requires variation of the flow of foamant as a function of the flow of water. A tank holds a quantity of liquid chemical foamant, and at least one pump has an inlet port coupled to the tank and an outlet port coupled to the conduit assembly. A hydraulic motor drives the pump and a valve is coupled with the hydraulic motor and a source of pressurized hydraulic fluid. A sensor is placed adjacent to the outlet port of the pump to feed back a signal representing the pressure of the water flowing through the conduit assembly, and a control system is responsive to the signal and to a flow rate of water flowing through the conduit assembly, producing a signal for controlling a speed at which the hydraulic motor drives the foamant pump. A feed-back loop is provided that monitors the flow rate and pressure of the water in order to control the foamant flow. The foamant outlet and the water/foam outlet pressure sensor are placed at substantially the same position in the flow so that the foam addition can be controlled to produce an accurately metered quantity of the liquid foamant in the water stream exiting the conduit assembly and to maintain the desired concentration.